1. Field of the Invention
The present invention relates to a thin-film semiconductor device and a liquid crystal display (LCD), and more particularly, to a thin-film semiconductor device and LCD provided with a plurality of thin-film transistors (TFTs) having different driving voltages.
2. Description of the Related Art
An image display such as an LCD or an organic EL display is used as a monitor of a notebook-type personal computer or a portable terminal device such as a cellular phone or a mobile device. For example, in an active matrix-type LCD, TFT is formed of poly-crystalline silicon by arraying a matrix of picture elements on a glass substrate and the TFT is driven by a driver circuit such as an external gate driver or a data driver. Therefore, an active matrix-type LCD displays an image thereon by controlling a orientation of the liquid crystal.
Recently, along with downsizing of the portable terminal device, in an image display a method has been developed for forming a driver circuit, an amplifier circuit, etc. integrally on a glass substrate without mounting them externally thereto. In the case of the LCD in which the driver circuit is mounted integrally, these circuits are formed simultaneously with the TFTs that are formed in the picture elements as switching elements.
These switching TFTs and driver circuit are constituted of two types of TFTs, N-channel and P-channel ones. The TFTs are driven under various conditions, including high and low driving voltages. In the case of the driver circuit, for example, the driving voltage for a scanning line is different from that for a data line, and the breakdown voltage required for the respective TFTs is different depending upon the driving voltage. To improve performance of a relevant LCD, it is necessary to form TFTs to be suited for the various driving voltages at a high throughput.
For this purpose, there has been a method available to alter a thickness of a gate insulating film in order to adjust the breakdown voltage of the transistors. FIG. 1 shows a cross-sectional view of two types of transistors having different film thicknesses of the gate insulating films on a silicon substrate. As shown in FIG. 1, a thickness of a gate insulating film 201 between a gate electrode 203 and a channel-active layer 205 of the transistor is different from that of a gate insulating film 202 between a gate electrode 204 and a channel-active layer 206 of the other transistor. A thickness of the gate insulating film of the right-side transistor in FIG. 1 is formed larger than that of the left-side transistor, in order to accommodate a higher drain voltage taking into account gate and drain breakdown voltages of the transistor.
However, in order to form the TFTs as shown in FIG. 1, it is necessary to form each gate insulating film separately for transistors including p-type and n-type having different breakdown voltage, thereby resulting in a complicated process. Moreover, the transistor having a higher drain voltage, as shown on the right side of FIG. 1, needs to have a Lightly Doped Drain (LDD) structure containing a lightly-doped drain region, thereby further complicating the process.